Piston rings for internal engines are required to have scuffing resistance and wear resistance when used with cylinder liners. To meet these requirements, the piston rings are plated with hard chromium, etc. on their outer peripheral sliding surfaces. However, because of the recent ban of using environmentally hazardous substance, hexavalent chromium used as a chromium source in chromium plating has become banned mostly in Europe.
Ni—W alloy plating films and thermally sprayed WC films have been developed as alternative films to the chromium plating film, but are not put into practical use because of disadvantages in performance and cost.
In the chromium plating film, research is under way to use less toxic trivalent chromium as a chromium source for a plating solution. Though trivalent chromium plating films as thin as 5 μm or less have been used so far for ornament and corrosion resistance, plating films as thick as 10 μm or more have not been industrially produced. As a result, research has been intensively conducted to provide thick plating films.
JP 56-112493 A teaches a chromium plating method using trivalent chromium and glycine as a chelating agent to produce a dense plating film as thick as several tens of micrometers.
JP 6-316789 A teaches a trivalent chromium bath containing wear-resistant, hard particles such as diamond particles, SiC particles, etc., and self-lubricating particles such as graphite, MoS2, etc., to produce a film having improved wear resistance, which can be as thick as 9 μm, more than conventional thickness of about 2 μm.
JP 9-95793 A teaches an industrially usable trivalent chromium plating bath containing boric acid and glycine in combination with an ammonium salt, to produce plating films as thick as about 50 μm. It also teaches that a heat treatment at about 400° C. can increase the as-plated hardness of about HV 700 to about HV 1500.
Not only to make thicker, but also to improve the corrosion resistance of trivalent chromium plating films, JP 2010-189673 A teaches a trivalent chromium plating bath in the form of an aqueous solution containing a water-soluble trivalent chromium compound, a conductive salt, a pH buffer, a sulfur-containing compound and an aminocarboxylic acid.
To prevent the generation of toxic gases such as a halogen gas, etc. due to the decomposition of solution components, for improved long-term storability and operation environment, WO 2012/133613 teaches a trivalent chromium plating bath in the form of an aqueous solution containing a trivalent chromium compound, a pH buffer, an aminocarboxylic compound, a sulfamate compound, and an aminocarbonyl compound.
However, when hard chromium plating films as thick as 5 μm or more are formed using these trivalent chromium plating baths, as large cracks as extending from their film surfaces to near substrate interfaces are likely formed. Further, when heat-treated at about 200-400° C. to have hardness HV of about 1000-1500, their crack opening widths increase as shown in FIG. 2, resulting in the partial peeling of plating films. Thus, there actually remain many problems to be solved for practical use.